1. Field of the Invention
The present invention is directed to a magneto-acoustic marker for use in an electronic article surveillance system, as well as to an electronic article surveillance system employing such a magneto-acoustic marker, and to a method for making such a magneto-acoustic marker.
2. Description of the Prior Art and Related Applications
Magneto-acoustic markers for electronic article surveillance (EAS) typically include an elongated trip of a magnetostrictive amorphous alloy which is magnetically biased by an adjacent strip of a magnetically semi-hard metal strip.
The typical requirements for such EAS markers are: a consistent resonant frequency at a given bias field which is primarily determined by appropriate choice of the length of the resonator, a linear hysteresis loop in order to avoid interference with harmonic systems, which is achieved by annealing the amorphous ribbon in a magnetic field perpendicular to the long axis of the resonator, a low sensitivity of the resonant frequency to the bias field, a reliable deactivability of the marker when the bias field is removed, and a (preferably) high resonant amplitude which persists for a sufficient time when the exciting drive field is removed.
Such resonators can be realized by choosing an amorphous Fe-Co-Ni-Si-B alloy which has been annealed in the presence of a magnetic field applied perpendicularly to the ribbon axis and/or a tensile stress applied along the ribbon axis. The annealing is preferably done reel to reel with typical annealing times of a few seconds at temperatures between about 300xc2x0 C. and 420xc2x0 C. Thereafter the ribbon is cut to oblong pieces which form the resonators. Such resonators, and a general background description of the physics and prior art relating to magneto-acoustic markers, are described in co-pending U.S. application Ser. No. 08/890,612 (xe2x80x9cAmorphous Magnetostrictive Alloy with Low Cobalt Content and Method for Annealing Same,xe2x80x9d G. Herzer), filed Jul. 9, 1997 and co-pending U.S. application Ser. No. 08/968,653 (xe2x80x9cMethod of Annealing Amorphous Ribbons and Marker for Electronic Article Surveillance,xe2x80x9d G. Herzer) filed Nov. 2, 1997. Both of these co-pending applications as assigned to the same assignee (Vacuumschmelze GmbH) as the present application, and the teachings of both of these co-pending applications are incorporated herein by reference.
Typical markers for EAS use a single resonator which is about 38 mm long, about 25 xcexcm and about 12.7 mm or 6 mm wide. The wider marker generally produces about twice the signal amplitude of the narrower marker, however, the narrower marker is more desirable because of its smaller size. A magnetostrictive marker employing two or more elongated strips of magnetostrictive ferromagnetic material, however, is described in U.S. Pat. No. 4,510,490. In the marker described therein, the strips are disposed side-by-side in a housing. The reason for using multiple resonator strips in this known marker is stated in the reference to be for the purpose of allowing the marker (i.e., the respective multiple strips thereof) to resonate at different frequencies, thereby providing the marker with a particular signal identity.
It is an object of the present invention is to provide a magneto-acoustic marker having reduced dimensions without degradation in performance.
More specifically it is an object of the present invention to provide a magnetostrictive amorphous metal alloy for incorporation in such a marker in a magnetomechanical surveillance system which can be cut into oblong, ductile, magnetostrictive strips which can be activated and deactivated by applying or removing a pre-magnetization field H and which in the activated condition can be excited by an alternating magnetic field so as to exhibit longitudinal, mechanical resonance oscillations at a resonance frequency Fr which, after excitation, are of high signal amplitude.
It is a further object of the present invention to provide such an alloy wherein only a slight change in the resonant frequency occurs given a change in the bias field, but wherein the resonant frequency changes significantly when the marker resonator is switched from an activated condition to a deactivated condition.
Another object of the present invention is to provide such an alloy which, when incorporated in a marker for magnetomechanical surveillance system, does not trigger an alarm in a harmonic surveillance system.
It is also an object of the present invention to provide a marker embodying such a resonator, and a method for making a marker suitable for use in a magnetomechanical surveillance system.
It is finally an object of the present invention to provide a magnetomechanical electronic article surveillance system which is operable with a marker having a resonator composed of such an amorphous magnetostrictive alloy.
The above objects are achieved in a method for making a magneto-acoustic EAS marker wherein two (or more) short oblong pieces of a narrow amorphous ribbon are disposed in registration in a housing to form a dual (multiple) resonator, with the respective resonant frequencies of the individual resonator pieces coinciding to within about +/xe2x88x92500 Hz and preferably within +/xe2x88x92300 Hz. This can be achieved by giving these pieces the same length and width, the same composition and the same annealing treatment. As a consequence it is advantageous to put two (or more) consecutively cut pieces (cut to the same length) together. Such an inventive magnetoelastic marker is capable of producing a resonant signal amplitude comparable to a conventional magnetoelastic marker of the prior art of about twice the width.
As used herein, placing the pieces xe2x80x9cin registrationxe2x80x9d means that the pieces are disposed one over the other with a substantial overlap, if not exact congruency. In any event, the term is intended to preclude a side-by-side arrangement as in the prior art.
For a dual resonator it is advantageous to choose an Fe-Ni-Co-base alloy with an iron content of more than about 15 at % and less than about 30 at % which is annealed in the presence of a magnetic field perpendicular to the ribbon axis and/or with a tensile stress applied along the ribbon axis. A generalized formula for the alloy compositions which, when annealed as described above, produces a dual resonator having suitable properties for use in a marker in a electronic article surveillance or identification system, is as follows:
xe2x80x83FeaCobNicSixByMz
wherein a, b, c, x, y and z are in at %, wherein M is one or more glass formation promoting elements such as C, P, Ge, Nb, Ta and/or Mo and/or one or more transition metals such as Cr and/or Mn and wherein
15xe2x89xa6axe2x89xa630
6xe2x89xa6bxe2x89xa618
27xe2x89xa6cxe2x89xa655
0xe2x89xa6xxe2x89xa610
10xe2x89xa6yxe2x89xa625
0xe2x89xa6zxe2x89xa65
14xe2x89xa6x+y+zxe2x89xa625
such that a+b+c+x+y+z=100.
In a preferred embodiment the resonator assembly consists of two ribbon pieces in registration, each ribbon piece having a thickness between about 20 xcexcm and 30 xcexcm, a width of about 4 to 8 mm and a length between about 35 mm to 40 mm.
The objects of the invention can then be realized in a particularly advantageous way by using the following refined ranges in the above formula
20xe2x89xa6axe2x89xa628
6xe2x89xa6bxe2x89xa614
40xe2x89xa6cxe2x89xa655
0.5xe2x89xa6xxe2x89xa65
12xe2x89xa6yxe2x89xa618
0xe2x89xa6zxe2x89xa62
15 less than x+y+z less than 20
such that a+b+c+x+y+z=100.
Examples for such alloys which are particularly suitable for a dual resonator which is about 6 mm wide and in a range between 35 mm to 40 mm in length are as follows. Suitable alloys which have been tested are represented by alloys Nos. 3 through 9 in Table I, namely Fe24Co12.5Ni45.5Si2B16, Fe24Co12.5Ni44.5Si2B17, Fe24Co13Ni45.5Si1.5B16, Fe24Co12Ni46.5Si1.5B16, Fe24Co11.5B16, Fe24Co11Ni48Si1B16 and Fe27Co10Ni45Si2B16. Various further compositions were tested in order to optimize the silicon and boron content in compositions having an iron content of 24 at %. Examples of these further compositions are Fe24Co12.5Ni45Si1.5B17, Fe24Co12.5Ni45Si2B16.5, Fe24Co12.5Ni45Si2.5B16, Fe24Co11.5Ni46.5Si1.5B16.5, Fe24Co11.5Ni46.5Si2B16 and Fe24Co11.5Ni46.5Si2.5B15.5. Similar compositions were also tested wherein the boron content was modified by about +/xe2x88x921 at % (starting from one of the above various further alloys) at the expense of the nickel content. If annealing is performed without tensile stress, a composition with a boron content which is lower by about 0.5 to 1 at % is more suitable.
Based on the above investigations, a preferred composition is Fe24Co11.5Ni46.5Si1.5B16.5, with Js=0.86T.
If the iron content is not held at 24 at %, other particularly suited compositions are Fe25Co10Ni47Si2B16 and Fe22Co10Ni50Si2B16. Lastly, from a mathematical analysis of the above samples and other experimental data, the following (and similar) alloy compositions are expected to be particularly suitable as well: Fe22Co12.5Ni47.5Si2B16, Fe24Co10.5Ni48Si2B15.5, Fe24Co9.5Ni49.5Si1.5B15.5 and Fe24Co8.5Ni51Si1B15.5. These alloys would be particularly suited because the cobalt content is further reduced, cobalt being the most expensive component of these alloys.
Based on the above investigations, an even further refined formula can be empirically deduced, which still falls within the above-cited, more general formulae. This further refined formula is as follows:
Fe24xe2x88x92rCo12.5xe2x88x92wNi45+r+v+1.5wSi2+uB16.5xe2x88x92uxe2x88x92vxe2x88x920.5
wherein r=4 to 4 at %, u=xe2x88x921 to 1, v=xe2x88x921 to 1 and w=xe2x88x921 to 4 at %.
With such alloy compositions, suitable magneto-acoustic properties can, for example, be achieved by continuously annealing (reel to reel process) in the presence of a magnetic field of at least about 800 Oe oriented perpendicularly to the ribbon axis and a tensile stress of about 50 MPa to 150 MPa with an annealing speed of about 15 m/min to 50 m/min and a annealing temperature ranging from about 300xc2x0 C. to about 400xc2x0 C. The annealing process results in a hysteresis loop which is linear up to the magnetic field where the magnetic alloy is saturated ferromagnetically. As a consequence, when excited in an alternating field the material produces virtually no harmonics and, thus, does not trigger alarm in a harmonic surveillance system.
Preferably the magnetic field during annealing is applied substantially perpendicular to the ribbon plane and has a strength of at least about 2000 Oe. This results in a fine domain structure with domain width smaller than the ribbon thickness and a resonant amplitude which is at least 10% higher than that of conventionally (transverse field) annealed ribbons.
Particular suitable alloy compositions have a saturation magnetostriction between about 8 ppm and 14 ppm and when annealed as described above, the hysteresis loop of the pieces put together to form the resonator assembly has an effective anisotropy field Hk between about 8 Oe and 12 Oe. Such anisotropy field strengths are low enough to provide the advantage that the maximum resonant amplitude occurs at a bias field smaller than about 8 Oe which e.g. reduces the material cost for the bias magnet and avoids magnetic clamping. On the other hand such anisotropy fields are high enough such that the active resonators exhibit only a relatively slight change in the resonant frequency Fr given a change in the magnetization field strength i.e. |dF/rdH| less than 750 Hz/Oe but at the same time the resonant frequency Fr changes significantly, by at least about 1.6 kHz, when the marker resonator is switched from an activated condition to a deactivated condition.
Usually an alloy ribbon optimized for a multiple resonator tag is unsuitable for a single resonator marker, and vice versa. By appropriate choice of alloy composition and heat treatment, however it is possible to provide an annealed alloy ribbon which is suitable for both a single and a dual resonator. Particular suitable alloys for this purpose have a saturation magnetostriction of about 10 ppm to 12 ppm and are annealed such that the anisotropy field Hk of the dual resonator is about 9 to 11 Oe. This object can be realized in a particularly advantageous way by applying the following ranges to the above formula:
22xe2x89xa6axe2x89xa626
8xe2x89xa6bxe2x89xa614
44xe2x89xa6cxe2x89xa652
0.5xe2x89xa6xxe2x89xa65
12xe2x89xa6yxe2x89xa618
0xe2x89xa6zxe2x89xa62
15 less than x+y+z less than 20
Examples of alloys which are particularly suitable for single and/or dual resonator having a width of about 6 mm and a length in a range between 35 mm to 40 mm are as follows. These alloys include alloy nos. 3 through 8 from Table I, namely Fe24Co12.5Ni45.5Si2B16, Fe24Co12.5Ni44.5Si2B17, Fe24Co13Ni45.5Si1.5B16, Fe24Co12Ni46.5Si1.5B16, Fe24Co11.5Ni47Si1.5B16 and Fe24Co11Ni48Si1B16. The following further compositions are also particularly suited for a dual and/or single resonator: Fe24Co13Ni45.5Si1.5B16, Fe24Co12.5Ni45Si1.5B17, Fe24Co12.5Ni45Si2B16.5, Fe24Co12.5Ni45Si12.5B16, Fe24Co11.5Ni46.5Si1.5B16.5, Fe24Co11.5Ni46.5Si2B16, Fe24Co11.5Ni46.5Si2.5B15.5, Fe24Co11Ni47Si1B16, Fe24Co10.5Ni48Si2B15.5, Fe24Co9.5Ni49.5Si1.5B15.5, Fe24Co8.5Ni51Si1B15.5 and Fe25Co10Ni47Si2B16.
A more refined formula based on the above examples for an alloy particularly suited for a dual and/or single resonator is
Fe24xe2x88x92rCo12.5xe2x88x92wNi45+r+v+1.5wSi2+uB16.5xe2x88x92uxe2x88x92vxe2x88x920.5w
wherein r=xe2x88x921 to 1 at %, u=xe2x88x921 to 1, v=xe2x88x921 to 1 and w=xe2x88x921 to 4 at %.
In order to obtain consistent properties along the ribbon length it is advantageous to perform the annealing with a feedback control. For this purpose the magnetic properties (e.g. the hysteresis loop) are measured after the ribbon has exited the furnace and the annealing parameters are adjusted if the resulting test parameter deviates from a predetermined value. This is preferably done by adjusting the level of the applied tensile stress, i.e. the tension is increased or decreased to yield the desired magnetic properties. This feedback system is capable of effectively compensating the influence of composition fluctuations, thickness fluctuations and deviations in the annealing time and temperature on the magnetic and magnetoelastic properties. The result are extremely consistent and reproducible properties of the annealed ribbon, which otherwise are subject to relatively strong fluctuations due to the afore-mentioned influences.
In order to correlate the measurement on a continuous ribbon with the resonator properties it is essential to correct the parameters for demagnetizing effects as they occur on the short resonator assembly. As an example, consistent resonator properties for a dual resonator are achieved when the sum of the anisotropy field of the continuous ribbon plus twice the demagnetizing field of a single resonator piece is kept at a constant, predetermined value, which preferably lies between about 8 Oe to 12 Oe.
In another embodiment of the present invention, more than two ribbon pieces are arranged in registration to form a multiple resonator, e.g. a triple resonator. Such a multiple resonator has the advantage that it produces even higher signal amplitudes. A generalized formula for the alloy compositions which, when annealed as described above, produce a multiple (i.e. at least triple) resonator having suitable properties for use in a marker in a electronic article identification system, is as follows:
FeaCobNicSixByMz
wherein a, b, c, x, y and z are in at %, wherein M is one or more glass formation promoting element such as C, P, Ge, Nb, Ta and/or Mo and/or one or more transition metals such as Cr and/or Mn and wherein
30xe2x89xa6axe2x89xa665
0xe2x89xa6bxe2x89xa66
25xe2x89xa6cxe2x89xa650
0xe2x89xa6xxe2x89xa610
10xe2x89xa6yxe2x89xa625
0xe2x89xa6zxe2x89xa65
15xe2x89xa6x+y+zxe2x89xa625.
such that a+b+c+x+y+z=100.
In a preferred embodiment the anisotropy of the amorphous alloy ribbon is controlled by applying a tensile stress during annealing with the following refined ranges in the above formula:
45xe2x89xa6axe2x89xa665
0xe2x89xa6bxe2x89xa66
25xe2x89xa6cxe2x89xa650
0xe2x89xa6xxe2x89xa610
10xe2x89xa6yxe2x89xa625
0xe2x89xa6zxe2x89xa65
15xe2x89xa6x+y+zxe2x89xa625
Examples for such alloys particularly suited for a 6 mm wide and a 35 mm to 40 mm long triple resonator are:
Fe46Co2Ni35Si1B15.5C0.5 and Fe51Co2Ni30Si1B15.5C0.5
A particularly suited example for a 6 mm wide resonator assembly consisting of 4 resonator pieces (about 35 to 40 mm long) is given by the composition Fe53Ni30Si1B15.5C0.5.
In general, the following compositions are preferred with respect to optimization of the silicon and boron content, and are also optimal for manufacturing ovens used by the Assignee (Vacuumschmelze GmbH) using an annealing process making simultaneous use of a perpendicular field and tensile stress, and these alloys are also the most promising candidates for further reducing the cobalt content. These preferred compositions are Fe24Co13Ni45.5Si1.5B16, Fe24Co12.5Ni45.5Si2B16, Fe24Co12.5Ni45Si2B16.5, Fe24Co11.5Ni46.5Si1.5B16.5, Fe24Co10.5Ni48Si2B15.5, Fe25Co10Ni47Si2B16, Fe24Co9.5Ni49.5Si1.5B15.5 and Fe24Co8.5Ni51Si1B15.5.
Lastly, it should be noted that typically as a result of ingot preparation, the resulting alloy in practice will contain carbon in an amount of up to about 0.5 at %, and correspondingly less boron.